There are many types of semiconductor electronic devices. Such devices, for example diodes and transistors, take advantage of the semiconducting nature of certain materials. Because of their various advantages, they have been steadily replacing conventional electrical parts in many applications over the last half century. Some of the advantages of semiconductor devices and circuits include their low cost, small size, and generally good reliability. They are especially suited for computing and memory operations and appliances using them have become ubiquitous. Personal computers, mobile telephones, and media players are just a few examples of applications that make use of semiconductor devices.
A semiconductor is a material that may be induced to act as either a conductor or as a non-conductor in a generally-controllable manner. Typical semiconductor materials include silicon, germanium, or combinations of the two that have been subject to a process known as ion implantation. Ion implantation, sometimes called ‘doping’, involves bombarding a semiconductor material, with ions of various elements such as boron and phosphorus. Using these two elements creates two different types of semiconductor materials, sometimes referred to as p-type and n-type, respectively. The basic difference between types of semiconductor materials involves their behavior under certain conditions, and specifically how they conduct electricity when properly induced. Selective ion implantation creates regions of these different types in the same piece of semiconductor material, often in this context called a substrate, and these regions may then be connected electrically to create a circuit. In general, the application of a certain stimulus, most often a small electrical charge, causes electrical current to flow in a predictable manner through and across the boundary between these regions. This allows, for example, the formation of switches and capacitors that have no moving parts.
A simple exemplary semiconductor device is shown in FIG. 1. FIG. 1 is a simplified block diagram illustrating a typical semiconductor diode 10. Diode 10 consists of a region of p-type semiconductor material 12 (sometimes called an anode) that is immediately adjacent to a region of p-type semiconductor material 14 (or cathode), the two regions meeting at p-n junction 13. The two regions are, in this example, portions of a larger silicon substrate (the remainder of which is not shown) that are formed by being separately implanted with boron and phosphorus ions, a process also known as doping. Contacts 11 and 15 have been couple to the p-type silicon 12 and the n-type silicon 14 so an electrical charge may be applied. When a forward bias, that is, a positive charge is applied to the anode, current flows across the p-n junction 13. Current will not flow in the opposite direction, however, even if the bias is reversed, giving the diode 10 its useful characteristic behavior.
A more sophisticated diode is illustrated in FIG. 2. FIG. 2 is a cross-sectional view illustrating a typical semiconductor gated diode 20. The gated-diode is so-called because it includes a gate structure that can be used to moderate the device's behavior. In this example, gated diode 20 includes a gate structure 25 that has been formed on substrate 21. Gate structure 25 includes a gate electrode 26 that is separated from the substrate 21 by a dielectric layer 27. The gate electrode 26 is made of a conducting material such as polysilicon or a metal, the gate dielectric 27 may be, for example, an oxide or nitride of silicon.
The substrate 21 in this example is made of a p-type material. In it have been formed regions of n-type material, referred to here as a source 22 and a drain 23. Source 22 and drain 23 define a channel 24 beneath the gate structure 25. In a transistor of similar design, such as a MOSFET (metal oxide semiconductor field effect transistor) the source and drain regions would be connected to different components, but in the MOS gated diode 20 they are coupled together as shown in FIG. 2. A separate lead is attached the gate electrode 26 so that a gate voltage Vg may be applied. The substrate 21, of course, serves as a ground. Note that because this device has structural similarities to a MOSFET, it may sometimes herein also be referred to as a MOSFET gated diode.
Like any electrical device, MOSFET gated diodes do not always function perfectly. One of the more significant problems encountered with devices such as the gated diode of FIG. 2 is junction leakage. Junction leakage is undesirable current flow, for example when in the reverse-bias or some other condition in which current flow is not supposed to occur. One solution has been use lower doping concentrations in the source and drain regions. Unfortunately, this can lower driving current and reduce overall device performance. Undesirable junction capacitance is also sometimes a problem.
As alluded to above, the electronic devices made of semiconductor materials are very small. Advances in fabrication technology, in fact, now enable the manufacture of chips, or separable sections of semiconductor wafers that are thin squares or rectangles much less than one centimeter on a side yet include over a million transistors, diodes, and other devices. The gate diode 20 of FIG. 2, for example, may be less than 100 microns wide and have a channel length (from the source region to the drain region) of less than 25 microns. As should be apparent, fabricating such small devices poses many challenges. In addition, the very short channel tends to give rise to the existence of undesirable ‘short channel effects’ (SCE), where the device does not behave in an ideal manner. At the same time, demand for even smaller devices is constantly driving designs, so that even small chips can be produced. It therefore becomes imperative to eliminate or at least minimize the unwanted SCEs and other similar problems.
Needed, then, is a gated-diode that can be fabricated in a small enough size to be useful in today's electronics devices without having to excessively lower the source and drain region doping concentration in order to prevent junction leakage.